As a result of the demand for lighter building materials, lightweight inorganic aggregates have been used in concrete as a replacement for natural gravel and sand. Such light-weight inorganic aggregates include very light materials such as vermiculite and pearlite, which are used in plastics and insulating concretes; materials such as scoria and pumice, which are typically used in fill concretes; and expandable materials such as shale, clay, slate or slag, which are typically used in structural concretes.
Lightweight aggregate from such expandable materials is produced by heating particles of the material to a temperature wherein the material becomes plastic and the evolution of gas within the particles causes the particles to expand. To assure proper expansion, the particles are generally heated slowly, typically in a rotary kiln. In such a process, the particles are introduced into the upper end of the kiln, which is inclined. As the kiln rotates, the particles move toward the discharge end. A burner at the discharge end creates a flow of hot air through the kiln which contacts and heats the particles. By the time the particles reach the discharge end of the kiln, they have expanded.
Although conventional processes effectively expand such particles, they tend to be very time consuming. For example, in one process, shale particles must be heated in a rotary kiln for up to about 25 minutes. Such processes are also expensive to operate as a result of the vast amount of energy needed to heat the particles for such an extensive period. Accordingly, a need exists for a quicker, less expensive process for expanding particles of a lightweight aggregate material.